The long-term goals of this project are to determine the mechanism of recombination of virus DNA with the host chromosome and to determine how intracellular virus DNA becomes infective (or matures). We have shown that a host mutant unable to support recombination between lambda and E. coli lacks a required protein. Integration of the phage genome into host chromosome appears to require a different form of the phage int protein than does excision. We have discovered an example of imprecise excision of the lambda chromosome that may shed light on the molecular mechanism of site specific recombination. We have characterized a new gene required for cutting and packaging of lambda DNA and we have shown that positioning of the right end of the DNA inside a lambda particle is a discrete and essential step in virus assembly.